This invention relates to a well-treating process for dissolving a siliceous material within a well borehole or a subterranean earth formation. More particularly, it relates to increasing the amount of siliceous material which can be dissolved by an aqueous mud acid solution of weak acid, weak acid salt and fluoride salt of the type described in the E. A. Richardson U.S. Pat. No. 3,889,753. That patent and/or the invention it describes is referred to herein as the U.S. Pat. No. 3,889,753 and its disclosures are incorporated herein by cross-reference.
In accordance with U.S. Pat. No. 3,889,753, a siliceous material is reacted relatively slowly with an aqueous hydrogen fluoride-containing acid by contacting the siliceous material with an aqueous solution of a fluoride salt, a weak acid and a weak acid salt in proportions providing a significant but low concentration of dissolved hydrogen fluoride. The concentrations are preferably correlated to provide a fluoride salt concentration equivalent to at least a clay-dissolving concentration of hydrogen fluoride, enough weak acid to convert at least an effective amount of the fluoride salt to hydrogen fluoride, and enough weak acid salt to provide a ratio of weak acid to weak acid salt that limits the rate of the acidization reaction.
The rates at which the mud acid solutions of U.S. Pat. No. 3,889,753 react with acid-reactive materials are kept low by limiting the concentration of hydrogen fluoride and ionized weak acid in those solutions. The limiting is provided by the buffering action of the mixtures of weak acids and weak acid salts that are dissolved in the solutions.
In an aqueous solution a weak acid ionizes to provide proportions of unionized weak acid, hydronium ions, and weak acid anions that satisfy the ionization constant for the acid. The reaction is an equilibrium reaction. If such a solution also contains the salt of a weak acid, the salt ionizes to yield additional weak acid anions, and that reduces the proportion of hydronium ions that are needed to satisfy the ionization constant for the acid.
In the mud acid solutions of U.S. Pat. No. 3,889,753, an adequate supply of fluroide anions is provided by the ionization of the dissolved fluoride salt. But, since the formation of a molecule of hydrogen fluoride requires a combination of a hydronium ion and a fluoride anion, and since the concentration of hydronium ions is kept low by the buffering action, the concentration of hydrogen fluoride is also kept low. They are both regulated by the buffering action of the weak acid and weak acid salt. In acidizing a siliceous material, the low concentration of hydrogen fluoride molecules severely limits the rate at which those molecules (of which a relatively few are distributed throughout the total volume of the solution) can come into contact with the siliceous material.
The buffer-regulated mud acids of U.S. Pat. No. 3,889,753 are adapted to have a relatively high pH at which they are uniquely insensitive to the presence of carbonate minerals. It is known that when a conventional mud acid is to be used where carbonate minerals are apt to be encountered, a pretreatment with an acid that is free of hydrogen fluoride is necessary. A conventional mud acid tends to release calcium ions (or the ions of other alkaline earth metals) from the carbonate minerals so that such ions react with hydrogen fluoride and form insoluble precipitates. But, since the mud acids of U.S. Pat. No. 3,889,753 can be buffer-regulated at a pH that is high enough to severely limit the rate of their reaction with carbonate minerals, they are substantially immune to such undesirable calcium ion induced-precipitation, with little or no pretreatment acid that is free of hydrogen fluoride.